Details of a love triangle involving the future husband of Baroness Thatcher's sister have been disclosed in the authorised biography of the Iron Lady.

A young Margaret Roberts was lavished with gifts by farmer Willie Cullen after the pair met in 1949, but the future prime minister decided he would be a better match for her older sister Muriel.

Details of Margaret Thatcher's first romance, which started while she was a student in wartime Oxford, are also included in the book.

Margaret Thatcher at the wheel of a tractor during a visit to her sister's farm Credit: PA/PA Archive

And extracts from the biography, published in the Daily Telegraph, show not entirely positive first impressions of her future husband Denis.In a letter to Muriel she wrote about "a Major Thatcher, who has a flat in London (age about 36, plenty of money)... not a very attractive creature - very reserved but quite nice".

Not a very attractive creature - very reserved but quite nice.

– Margaret Thatcher's first impressions of her husband Denis

The book by Charles Moore draws on letters from the future Tory leader to her sister, mostly from 1940 to the early 1950s, sharing her romances and her flirtations with politics.

Her first relationship was with Tony Bray, an army cadet she met through the Oxford University Conservative Association, but the relationship fizzled out when his military training took him away from Oxford.

Speaking about a night when the pair attended a ball at the Randulph hotel, 60 years later, Mr Bray broke down in tears, "it was a very special evening," he said.

Margaret Thatcher, shepherds crook in hand, feeds a cow during a visit to her sister's farm Credit: PA/PA Archive

As a research chemist at BX Plastics at Manningtree, Essex in 1949, she met Mr Cullen, she had also been selected as the Conservative candidate for Dartford.

Keen to introduce her new friend to her sister, despite going on a series of dates with him, she wrote: "Went to the flicks yesterday with my farmer friend and got him all primed up to meet you sometime.

"I showed him the snapshot of you and I [sic] together - and he said he could scarcely tell the difference so I should think we could easily substitute me for you. When can you come down for a weekend?"

I showed him the snapshot of you and I [sic] together - and he said he could scarcely tell the difference so I should think we could easily substitute me for you. When can you come down for a weekend?

– Margaret Thatcher on her sister's future husband

A few weeks after her relationship had begun with Willie, she introduced him to her sister, despite his attempts to woo the future prime minister, giving her "frightfully expensive" Crepe de Chine scent and a "very nice" handbag with her initials on it.

It was at this time that Margaret began seeing Denis Thatcher, writing to her sister she talked about going to a Rotary Ball "with a chap called Denis Thatcher who is managing director of the Atlas paint works in Erith... He's all right - but is most unpopular with his men. He's far too belligerent in dealing with them and they naturally don't like it".

Margaret Thatcher, 26, at her wedding to 36-year-old Denis Thatcher at Wesley's Chapel, in London Credit: PA/PA Archive

By January 1950, Mr Cullen's attentions had begun to shift towards Margaret's sister Muriel, discussing the situation over the phone, Margaret said: "I told him from henceforth that I would 'in law' only be taking a sisterly interest in future".

"He seemed quite satisfied and is quite pleased with 'future prospects'," she told her sister.

Just over a month later, on Valentine's Day, Muriel Roberts and William Cullen announced their engagement, and were married in April that year, with Lady Thatcher as their only bridesmaid.

That same month, the future prime minister failed in her attempt to win the safe Labour seat of Dartford at the election.